The present invention relates to the treatment of well servicing fluids such as workover and completion fluids used in the oil and gas industry. More particularly, the present invention relates to the removal of certain heavy metals from generally solids-free brines used as well servicing fluids.
Clear, solids-free brines are commonly used as completion and workover fluids in the oil and gas industry. Such brines generally comprise aqueous solutions of heavy metal salts such as calcium chloride, calcium bromide, zinc chloride, zinc bromide, and various mixtures thereof, and may or may not contain viscosifiers. Typically, such brines are devoid of weighting agents, e.g. barite, and thus are commonly referred to as being clear and solids-free. Because the heavy metal salts and hence the brines from which they are made are relatively expensive, it is desirable to treat used brines to remove unwanted solids and impurities so that they can be reused in further completion and workover operations. It frequently happens that the used brines become contaminated with certain heavy metals, e.g. iron, which renders them unsuitable for use in either completion or workover operations. While this contamination may occur while the brine is being used, it also is possible for the brine to be contaminated prior to usage from storage tanks, transport vessels and the like.
The presence of heavy metals, more specifically heavy metal ions dissolved in the brines, poses a problem inasmuch as such ions, particularly under the conditions existing in the borehole, frequently precipitate out as the oxide or hydroxide thereof when they contact the formation or mix with formation water. This can result in formation damage. For example, the precipitated heavy metal oxides and/or hydroxides can build up filter cake on the wall of the borehole preventing or severely reducing production from the producing zone. Accordingly, a method for removing such undesirable heavy metals from the brines prior to use or reuse is desirable.